1. Field of the Invention
The present invention relates to ring laser gyroscopes and more particularly to ring laser gyroscopes which have bias errors resulting from gas flow within the gas medium.
2. Description of Prior Art
In ring laser gyroscopes, a pair of counter-rotating monochromatic light beams are propagated within a sealed cavity about a closed-loop optical path. The two beams travel in opposite directions around the closed-loop optical path. As the gyroscope cavity rotates in inertial space, the two oppositely traveling beams travel unequal path distances. A component of each light beam is extracted at a single point within the cavity, the point being referenced to the ring laser gyro housing where each of the two components are focused on a suitable detector. The two beams reach the detector having a relative frequency difference resulting from the rotational rate of the gyroscope housing in inertial space. The relative frequency difference is detected as an electrical beat signal which is then electronically interpreted to indicate the direction and inertial rate of rotation of the ring laser gyro housing about the gyro's sensitive axis.
Lasing is typically obtained by applying a DC current source having a voltage sufficient to sustain a discharge in the active gas medium in the gain bore. The discharge is supported between spaced-apart electrodes. The discharge, or plasma excitation current produces a gas flow of ionized gas between oppositely polarized electrodes in the optical cavity. It has long been known that this gas flow tends to induce a bias error in the detected signal of a ring laser gyro. By bias error, we mean a frequency difference between the two beams which are not in proportion to the rotation rate such as: a frequency difference in the absence of rotation, or a change in the frequency difference for a specific rotation rate. Ring laser gyros that use direct current excitation are therefore typically provided with a balanced electrode structure wherein two electrodes of one polarity are symmetrically disposed at each end of the gyro's gain bore about a single electrode of the opposite polarity positioned at the center of the gain bore. This structural arrangement produces two equal and symmetrical plasma flows of ionized gas thereby substantially reducing the bias errors in the detected beat signal.
The active gas medium in a ring laser gyro is typically excited by a DC current source producing a voltage drop between the cathode and each of the two respective anodes. The current source delivers relatively balanced currents to each respective anode through respective active gas medium segments. The gas flow produced in response to the current flow, contributes to a return gas flow between electrodes of opposite polarity. The total ring laser gyro bias error in the detected beat signal is related to the magnitude of the pressure difference between electrodes of opposite polarity.
A prior pending U.S. Patent Application titled "Ring Laser Gyro Gas Bypass", Ser. No. 496,166, filed May 19, 1983 and having the same inventor and assignee, is pertinent. It provided a partial solution to bias errors by providing a gas bypass between electrode locations of opposite polarity through apertures in the gain bore in the lasing path. In this prior invention, the gas bypass operates to control the return gas flow by providing an alternate path for the return gas flow. The alternate path also serves to reduce the pressure differential between electrode locations of opposing polarity.